Turning a 1955 Chevy Into a Modern Hotrod - No Jive For Your '55

To further enhance the power production of the five upgrades we added in Part 1, a set of AFR Magnum 305 heads was chosen to top off the 454. Here, Joey D's 55 Bel Air (the No-Jive-Five-Five) is strapped down on SLP's dyno to see what the heads are worth.

We add a set of AFR 305 heads to our 454 to complete its transformation into a 21st-century big-block.

Last month, we tested five different key components to update an old-school 454 Rat in Joey Diorio's '55 Chevy. Once we changed from the old-tech hot rod parts to the five new pieces, we realized a total gain of 73 hp and 87 lb-ft of torque to the rear wheels.

These were not off-the-wall, pie-in-the-sky parts, just the latest off-the-shelf components from the aftermarket. We did this to illustrate how much more horsepower may be lurking in your prehistoric big-block. The first of the five changes was replacing the 20-year-old-design hydraulic flat-tappet cam with a new Lunati Voodoo solid street roller. The gains were such that the old 750 Holley double-pumper was no longer up to the task of feeding the 454. We ditched it for a new Holley 950 HP carburetor.

3/19

These aluminum AFR cylinder heads (as-cast Magnum 305s) will improve airflow, power, driveability, and efficiency over the old iron castings. We upgraded to roller springs (250 pounds closed, 600 pounds open) with titanium retainers. Standard equipment includes ARP rocker studs and guideplates to top off these beauties.

Going from the stock 14 volts to 18 volts of ignition power using Performance Distributors' Mini VIP gave us another 7 hp and 4 ft-lb. Switching to a Wilson Manifolds four-hole tapered carb spacer in place of a plain four-hole spacer gave us four more ponies. The last swap was getting rid of the '70s-era Weiand Team G single-plane intake for a recently redesigned version (plus 14 hp and 7 ft-lb). These five alterations also dropped the '55's e.t. by more than a second on the strip.

Today's street engines often make more power per cubic inch than '60s and '70s race engines. This is due to the technically advanced designs and manufacturing of today's automotive components. The next step to complement the aforementioned changes was swapping on a set of state-of-the-art cylinder heads. Recent offerings in cylinder heads just blow away those of yesteryear, even if those old castings received a thorough porting job by an experienced and skilled technician.

Our choice for this upgrade was a set of Airflow Research's aluminum 305cc Magnum heads. The 454's stock iron castings were the good (back in the day), high-performance, rectangular port heads. The stock rectangular port castings may have larger intake runners (315-320cc), but they offer less airflow than the AFR's smaller 305cc runners. The AFR 305s provide maximum airflow with minimum port volume for added velocity. A smaller port that flows more air is better for driveability (part-throttle response) and power production throughout the rpm range. These AFR heads demonstrated huge flow improvements at every lift value, not just at peak.

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Goin' Flowin'After receiving the AFRs at our Jersey office, we brought them to SLP Engineering in Toms River, New Jersey, for flow bench testing. We had a pair of ported rectangular port heads on hand to flow-test against the AFRs. After Joey D removed the bone-stock heads from his '55, we benefited from a three-way head-flow shootout. The AFR's average airflow numbers from .200- to .700-inch lift were 48 cfm more than the stock rectangular port heads and 44 cfm more than the ported heads. At .500-inch lift the as-cast AFR 305s produced 62 cfm more airflow than the stock castings and 54 more cfm than the ported units. (See the flow charts on page 92.) AFR now has 315 CNC-ported BBC heads that offer even more airflow gains than the tested as-cast 305s.

The AFRs are stock replacement heads with a few considerations. You'll need longer pushrods and 1-inch-longer head bolts for the four bottom exhaust boltholes. The exhaust ports are raised W-inch, which can cause header clearance issues (like on our Tri-Five with the "605" power steering box). We tried 2-inch headers made to fit a stock BBC in a Tri-Five, but they ended up hitting the block and the transmission bellhousing area. Ironically, we had to reuse the old '80s Blackjack 1 Y-inch headers that fit snugly next to the "605" steering box. For our new combination we should have been able to use 2- or 2 V-inch headers for the new, better-breathing heads. Had we been able to find such headers that fit with the power steering (Joey didn't want to do any cutting on the '55), we would have seen even more dramatic results.

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Turning a 1955 Chevy Into a Modern Hotrod - No Jive For Your '55

To further enhance the power production of the five upgrades we added in Part 1, a set of AFR Magnum 305 heads was chosen to top off the 454. Here, Joey D's 55 Bel Air (the No-Jive-Five-Five) is strapped down on SLP's dyno to see what the heads are worth.

These aluminum AFR cylinder heads (as-cast Magnum 305s) will improve airflow, power, driveability, and efficiency over the old iron castings. We upgraded to roller springs (250 pounds closed, 600 pounds open) with titanium retainers. Standard equipment includes ARP rocker studs and guideplates to top off these beauties.

The heart-shaped CNC chamber measured 119cc before optional angle milling brought ours to 112cc. The fully CNC-machined chamber enhances flow while creating a faster, more efficient combustion for added power. Looking at these castings side by side, the iron unit (left) shows its outdated and less efficient 118cc combustion chamber with 2.19/1.88 valves. The aluminum piece sports its modern and effective CNC chamber with 2.25/1.88 valves.

Here's a look at the 9:1 454 Rat/mule motor with the antiquated iron heads removed while awaiting its new aluminum units. This '70s replacement/crate motor has been serving well for more than 20 years.

Most gearheads are familiar with the '60s/'70s BBC head castings. When a BBC head displays "HIGH-PERF," it'll have rectangular ports, while castings with the "PASS" designation will have oval ports. The 990 castings (like this one fresh off the Rat) were equipped on the hundreds of 454 crate motors sold back in the 1970s and 1980s.

Both the iron (bottom) and aluminum casts are obviously rectangular port BBC heads. The stock iron castings typically have 320cc intake runners. The smaller 305cc AFRs have higher air velocity with much better flow. AFR has done lots of R&D to design its heads with a smaller port volume, better port shape, combustion chamber, and a special five-angle valve job to produce a desirable head package.

Looking closely at the deck of both heads, it's noticeable the new castings have a thicker deck surface with raised (W-inch) exhaust ports. The raised ports flow better to produce more power but can cause header clearance issues. Be sure to purchase the appropriate headers for these recent offerings with their raised exhaust ports.

On SLP's SF-1020 SuperFlow flow bench, the AFRs, stock 990s, and ported 291s went head-to-head in a flow comparison test. The iron and aluminum heads were flowed with a realistic bore size fixture of 4.280-inch (typical 427 or 454, .030 over). Most BBC heads are flowed using a 4.600 bore. A larger bore unshrouds the valves to help show better flow numbers. Surprisingly, our flow bench results (with the smaller bore) were similar at the lower (.200-.400-inch) lifts but not as much at high lifts (.500-.700-inch). Still, the AFRs flowed so much more cfm at all lift values (see the charts) than the old castings that we couldn't wait to see the difference on the dyno.

We used Lunati's pushrod-length checker (PN 80121) to determine the proper-length pushrods. The AFRs will usually require pushrods anywhere from .080-.200-inch longer than stock. This must be checked for correct rocker arm geometry. Carefully follow the instructions.

The new heads were torqued in three steps to 35, 55, then 75 ft-lb using ARP head bolts. Notice the adjustable-length pushrod and rocker in place and ready to help us determine the correct pushrod length. Again, be certain to check proper pushrod length due to varied deck heights, cam base circle, head milling, etc.

The position of the rocker's wheel is important. The wheel needs to roll across the top of the valve tip. At midlift the bottom of the roller wheel should be at the middle of the valve tip. Seen here, the wheel is at the first third of the valve tip at zero lift. Also, be sure there's at least .030-inch rocker to retainer clearance. At full lift, check that coil bind clearance is more than .060-inch.

Tri-Fives with the popular "605" power steering box will encounter header clearance issues using the latest aluminum heads with raised exhaust ports. We tried 2-inch headers made to fit with stock heads, but they would not fit with our new castings. The 2-inch tubes also rested against the bellhousing area of the block and trans, creating a N-inch gap between the header flange and the head. AFR's Tony Mamo recommended we use 2-inch (minimum) or 2 V-inch tubes for the higher-flowing heads to produce optimum power.

Ironically, it all went back together with the '80s Blackjack 1Y-inch headers (no longer produced). Even though we couldn't use the right size headers, at least this would be a true head swap-only test-for now. Perhaps in a future installment we'll try larger tubes.

We were stoked to see the AFR heads deliver peak-to-peak gains of more than 50 rear wheel horsepower. The stock heads produced max hp at 5,400 rpm, while the AFRs were still climbing at 6,200 rpm when the tach light lit, causing us to automatically lift. At 6,100-6,200 rpm the AFRs were making nearly 80 more rear wheel horsepower. On the first few pulls we noticed the A/F mixture was too lean at 13.5 (12.9 to 13.0 is usually ideal). We came armed and ready for tuning with boxes of Holley tuning parts.

Next, we replaced the four high-speed air bleeds (they're the four inner screws; the four outer screws are for the idle/cruise circuit). The stock #32s were replaced with #28s, which made the A/F mixture safer but a little too rich at 12.6. Throttle response was better and power was maintained, but we felt it was counterproductive to try further tuning on this hot and humid (90-degree) August day. At that we called it a day and went home for supper

The as-cast AFR 305s were definitely worth the effort and expense after realizing a peak-to-peak power jump of more than 53 rear wheel horsepower. At 6,200 (we let up at 6,200 for the baseline) there was an additional 82 rear wheel horsepower. Starting with the Part 1 baseline (329 rear wheel horsepower) to after testing the six upgrades (cam, intake, carb, spacer, 18 volts, and heads), the 454 picked up 126 horsepower to the back tires (455 total). The total combination (especially after the head swap) produced a powerband that kept pulling up the rpm scale after the stock heads had power dropping.

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